The Skeleton Warrior was a short-lived creature in the days of Advanced Dungeons & Dragons, appearing in the Fiend Folio. Created to make up for the fact that monsters couldn't "level up" the way adventurers could in those days, the Skeleton Warrior exists to keep the skeleton relevant for longer.
Fluff-wise, a Skeleton Warrior is an undead Fighter of considerable level, brought back from the dead as an intelligent, self-aware skeletal undead by an evil necromancer or demigod, who controls them by trapping their soul in a golden circlet. This circlet can be used to control the Skeleton Warrior, but it will always be watching for a chance to turn on its master and kill them; only by obtaining the circlet can the Skeleton Warrior's soul be released to the afterlife.
Standing as an interim step between skeleton and Death Knight, Skeleton Warriors are extremely tough. Their weapons hit as if they were +3 weapons, they are immune to non-magical weapons themselves, and they have a 90% resistance to magic spells. They exude an aura of fear that causes any creature with less than 5 hit dice to flee at the sight of them, and they are immune to clerical turning effects.
The Skeleton Warrior's legacy lives on mostly in the Skeletal Champion template for Pathfinder.
A skeleton warrior named Dekion appeared in an early episode of the D&D Cartoon.
There was also a Science Fantasy cartoon series in the early 90s (when else?) called "Skeleton Warriors" and it was metal as fuck, with heroic knights battling a band of evil warriors who had attempted to seize a magical artifact, only to be transformed into ghastly living skeletons with dark magical powers. Like most 80s-to-mid-90s toons, it had an awesome theme song. You can check it out here.